1. Field of the Invention
The invention relates to a process for removing fluorine. More particularly, it relates to a process for removing fluorine during the production of fertilizer from nitric acid, sulfuric acid and a phosphate source, e.g. from the raw ore or from slimes or other by-products from phosphate ore beneficiation and ammonia. Ammonium sulfate can be used instead of sulfuric acid.
2. Discussion of the Prior Art
A common method of producing both suspension and solution fertilizers containing nitrogen and phosphate is by ammoniation of phosphoric acids, especially wet-process phosphoric acid. As is taught in U.S. Pat. No. 4,066,432, there are several major routes from phosphate rock to such fluid fertilizers.
One route is the use of the direct process, using the heat of ammoniation to convert wet-process acid to an ammonium polyphosphate melt. A second route involves the direct two-stage ammoniation process as taught in U.S. Pat. No. 3,813,233 for the production of orthophosphate suspension by ammoniation by wet-process acid to from concentrated N-P.sub.2 O.sub.5.
In many processes employing fluorapatite ores hydrofluorsilicic acid is a by-product of the manufacture of superphosphates. Fluorine is usually removed as volatile hydrogen fluoride and silicon tetrafluoride when phosphate rock is dissolved in hot acid. These fluorine compounds are then scrubbed out from the gases. U.S. Pat. No. 4,062,929 teaches producing HF by reacting H.sub.2 SiF.sub.6 with ammonia to give ammonium fluoride which is fed with partly recycled alkali metal fluoride to a continuously operated reactor where a bifluoride of an alkali metal is formed in aqueous solution. The solid alkali metal bifluoride is heated to produce HF.
There is an extensive discussion of the chemistry of recovering phosphates in an article by D. R. Peck entitled "The Utilization of Phosphorus Minerals", found in Mellor's Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. VIII, Supplement III, p. 71 (1971).